Numerous physiological processes rely on a proper balance between generation and disposal of bodily fluids. As an example, within the body, the brain and spinal cord are bathed in cerebrospinal fluid, which helps maintain their proper operation. Large cavities within the brain, the ventricles, produce and reabsorb the cerebrospinal fluid so that it is regularly replaced for healthy operation of the body's neurological system. However, in some cases an imbalance occurs between the rate of fluid production and the rate of reabsorption. This can lead to hydrocephalus, a condition wherein the brain's ventricles become enlarged with cerebrospinal fluid, pressing brain tissue against the skull. This can lead to serious neurological problems, and potentially death.
The most effective treatment for hydrocephalus is surgical insertion of a shunt, a valve which vents excess cerebrospinal fluid from the brain. A neurosurgeon makes an estimate of the amount of flow required to relieve hydrocephalus, and selects a shunt having the desired flow capacity. A flap is cut in the scalp, a small hole is drilled in the skull, and a catheter is inserted to pass into a ventricle of the brain. The catheter is then connected to the shunt beneath the scalp. Another catheter is attached to the outlet of the shunt and is usually tunneled into the skin, down the neck, and into the peritoneal (abdominal) cavity. The scalp is then sewn shut over the shunt, which vents the excess cerebrospinal fluid to the peritoneal cavity for absorption. When a shunt is installed in this manner, it is generally referred to as a ventriculoperitoneal (VP) shunt. Shunts can also be installed to drain from the ventricles into the venous system or other receptive body cavities; further, shunts are also sometimes used to regulate the flow of liquids from structures other than the brain.
Common shunts are subject to numerous problems which designers have long sought to overcome, such as problems with tissue ingrowth and fouling, plaque buildup, and size and cost issues. Control of shunt valve response characteristics (i.e., flow volume vs. pressure characteristics) remains a major issue, with common problems being that response characteristics may undesirably change over time, and it is often difficult to easily adjust the response characteristics of a shunt to suit the needs of individual patients. Another common problem is the tendency for a shunt valve to open once a static threshold pressure difference is present, but then remain partially open after this static pressure difference is relieved. This effect arises owing to the dynamic pressure of fluid flow, and can (for example) result in unnecessary drainage at of cerebrospinal fluid at lower than desired pressures in the ventricles.
Another disadvantage of many shunts is that they operate in the manner of a common check valve, wherein their flow is dependent on the pressure difference across the valve between its inlet and outlet. When the pressure difference exceeds some threshold level, the valve opens to allow cerebrospinal fluid to drain from the brain. This arrangement can lead to the problem of siphoning: since opening of the valve is in part dependent on the pressure at the valve outlet, low pressure on the outlet side may cause the valve to open even if the pressure on the inlet side is not indicative of excess cerebrospinal fluid pressure in the ventricles. Thus, if a patient repositions his/her head with respect to the rest of his/her body—for example, by standing up from a supine position—decreased pressure in the peritoneal cavity can increase the pressure difference to such an extent that the valve opens and unnecessarily drains cerebrospinal fluid.
A further disadvantage of many shunts is that they allow reversible flow, i.e., they can allow backflow from the peritoneal cavity to the brain if the (downstream) pressure in the peritoneal cavity should for some reason exceed the (upstream) pressure in the brain by the same threshold pressure difference (though in this case the pressure difference is reversed between the upstream and downstream sides). This is naturally undesirable since it can cause a sudden increase in fluid pressure on the brain, whereas a sudden increase in fluid pressure in the peritoneal cavity is generally not critical.